Sir Alec Guinness (born Alec Guinness de Cuffe; 2 April 1914 – 5 August 2000) was an English actor. In the BFI listing of the 100 most important British films of the 20th century, he was the single most noted actor, represented across nine films — six in starring roles and three in supporting roles — including five directed by David Lean and four from Ealing Studios. He won an Academy Award, a BAFTA, a Golden Globe and a Tony Award. In 1959, he was knighted by Queen Elizabeth II for services to the arts. He received a star on the Hollywood Walk of Fame in 1960, the Academy Honorary Award for lifetime achievement in 1980 and the BAFTA Academy Fellowship Award in 1989.

Guinness began his stage career in 1934. Two years later, at the age of 22, he played the role of Osric in Hamlet in the West End and joined the Old Vic. He continued to play Shakespearean roles throughout his career. He served in the Royal Naval Reserve during the Second World War and commanded a landing craft during the invasion of Sicily and Elba. Along with Laurence Olivier, John Gielgud and Ralph Richardson, he was one of the great British theatre actors who made the transition to films after the war, making his name in six Ealing comedies, starting in 1949 with both A Run for Your Money and Kind Hearts and Coronets (in which he played eight different characters). He went on to lead roles in 1951 with The Man in the White Suit and The Lavender Hill Mob (for which he received his first Academy Award nomination for Best Actor), then in 1955 with The Ladykillers, and culminating in 1957 with Barnacle Bill.

Guinness collaborated six times with director David Lean: as Herbert Pocket in Great Expectations (1946); Fagin in Oliver Twist (1948); Col. Nicholson in The Bridge on the River Kwai (1957), for which he won both the Academy Award and the BAFTA Award for Best Actor; Prince Faisal in Lawrence of Arabia (1962); General Yevgraf Zhivago in Doctor Zhivago (1965); and Professor Godbole in A Passage to India (1984). In 1970, Guinness played Jacob Marley's ghost in Ronald Neame's Scrooge. He also portrayed Obi-Wan Kenobi in George Lucas's original Star Wars trilogy, which brought him further recognition; for his performance in the original 1977 film, he was nominated as Best Supporting Actor for the Academy Award and Golden Globe. Guinness's later life was closely associated with his definitive depiction of the leading role of George Smiley in the BBC television series Tinker, Tailor, Soldier, Spy and Smiley's People by John le Carré.

Guinness was born Alec Guinness de Cuffe at 155 Lauderdale Mansions South, Lauderdale Road, in Maida Vale, London. His mother's maiden name was Agnes Cuff; she was born on 8 December 1890 to Edward Charles Cuff, a sometime lifeguard at Bournemouth who had served in the Royal Navy, and Mary Ann, née Benfield, of a family of stonemasons and publicans. On Guinness's birth certificate, his mother's name is given as Agnes de Cuffe; the infant's name (where first names only are placed) is given as Alec Guinness, and there are no details for the father.

The identity of Guinness's father has never been officially confirmed. Agnes Cuff had worked at Cowes on the Isle of Wight as a barmaid at the Royal Yacht Squadron clubhouse at the time of the Cowes Regatta in 1913, which was attended by several members of the Guinness family including Edward Guinness, 1st Earl of Iveagh, and his sons Ernest and Walter. Members of the Guinness family claimed a "distinct resemblance" between Alec and one or other of the Guinnesses at Cowes that year; Honor Guinness, who made Alec's acquaintance in 1950 and invited him to tea with "his cousin", later visiting Alec's family with photo albums and diaries to point out the similarities she perceived, believed either her uncle Ernest or his brother Walter ("a celebrated seducer") was Alec's father, while her cousin Lindy considered Alec closely resembled her father, Loel.

From 1875, under English law, when the birth of an illegitimate child was registered, the father's name could be entered on the certificate only if he was present and gave his consent. Guinness himself believed that his father was a Scottish banker, Andrew Geddes (1861–1928), who paid for Guinness's boarding-school education at Pembroke Lodge, in Southbourne, and Roborough, in Eastbourne. Geddes—who with a "round face and sticking-out ears" bore a resemblance to Guinness and believed himself to be his father— occasionally visited Guinness and his mother, posing as an uncle. Guinness's mother later had a three-year marriage to a Scottish army captain named Stiven, whose behaviour was often erratic or even violent.

Guinness first worked writing advertising copy. His first job in the theatre was on his 20th birthday (2 April 1934), while he was a student at the Fay Compton Studio of Dramatic Art, in the play Libel, which opened at the old King's Theatre, Hammersmith, and then transferred to the West End's Playhouse, where his status was raised from a walk-on to understudying two lines, and his salary increased to £1 a week. He appeared at the New Theatre in 1936 at the age of 22, playing the role of Osric in John Gielgud's successful production of Hamlet. Also in 1936, Guinness signed on with the Old Vic, where he was cast in a series of classic roles. In the later 1930s, he took classes at the London Theatre Studio. In 1939, he took over for Michael Redgrave as Charleston in a road-show production of Robert Ardrey's Thunder Rock. At the Old Vic, Guinness worked with many actors and actresses who became his friends and frequent co-stars in the future, including Gielgud, Ralph Richardson, Peggy Ashcroft, Anthony Quayle, and Jack Hawkins. An early influence was film star Stan Laurel, whom Guinness admired.

Guinness continued playing Shakespearean roles throughout his career. In 1937, he played Aumerle in Richard II and Lorenzo in The Merchant of Venice under the direction of John Gielgud. He starred in a 1938 production of Hamlet which won him acclaim on both sides of the Atlantic. He also appeared as Romeo in a production of Romeo and Juliet (1939), Malvolio in Twelfth Night, and as Exeter in Henry V in 1937, both opposite Laurence Olivier, and Ferdinand in The Tempest, opposite Gielgud as Prospero. In 1939, he adapted Charles Dickens's novel Great Expectations for the stage, playing Herbert Pocket. The play was a success. One of its viewers was a young British film editor, David Lean, who later had Guinness reprise his role in Lean's 1946 film adaptation of the novel.